The investigations proposed deal with three classes of putative transmitters and or neuromodulators in mammalian retinas and are concerned with characterizing their distributions, receptors, influences on cyclic AMP levels in target neurons, possible interactions between systems utilizing these molecules, and gaining clues as to their physiological significance. The first of these is adenosine, which we have found to influence cyclic AMP levels in the rabbit retina and to be taken up by specific neurons in the rabbit and mouse retinas. The second class is indoleamines. We have found in long inbred strains of mice that tryptamine or serotonin but not melatonin prevents the accumulation of a light- sensitive pool of cyclic AMP in what our evidence suggests to be photoreceptors. In addition, it has been reported that long inbred mouse lines but not mouse lines recently inbred from wild strains appear to lack pineal melatonin and enzymes involved in its biosynthesis. Unless regulatory genes are involved, this may also hold for their retinas, and an opportunity may be at hand to help define the role of melatonin in mammalian retinas. The third class of transmitter/modulators is dopamine, which when applied to mouse retinas in micromolar concentrations, appears to counter the above effects of tryptamine or light on a light-sensitive pool of cyclic AMP. Adenosine is also reported to interact with dopamine release.